In installations for producing metal products in a strip form, such as thin rolled sheet metal, the product is subjected, after rolling, galvanizing, tinning or another process, to a certain number of finishing operations, to eliminate certain defects. In particular, the strip is passed through a leveller stand which is intended to eliminate twisting caused by the fact that the metal fibers are not of a regular length over the width of the sheet metal. To this end, the sheet metal is advanced on rollers defining an undulating path and subjected to tension, so as to produce elongations by means of traction deflection.
A leveller therefore generally consists of a levelling stand disposed between two tension blocks, one located at the entrance and one at the exit, within which stand are mounted a plurality of levelling rolls, the axes of which are disposed in a staggered arrangement with respect to one another on different levels, so as to define an undulating path.
The levelling stand may also be combined with another device which is located upstream thereof and which has a different function such as, for example, a rolling stand called a "skin-pass" which effects a slight elongation.
By way of example, a conventional type of leveller is represented diagrammatically in FIG. 1. The leveller conventionally comprises two tension blocks at the entrance 11 and at the exit 12 between which a levelling stand 10 is disposed.
Each tension block 11 or 12 comprises a certain number of rollers 13 and 17 which are intended to keep under tension the strip 3 that is being advanced and to confer on it an elongation by appropriate means. The levelling stand 10, disposed between the tension blocks, generally consists of two or four active levelling rolls 2 on which the strip 3, advancing in directions perpendicular to the axes of the rolls, is subjected to traction-deflection distortions in the elastoplastic range.
To this end, the active rolls preferably have a very small diameter relative to their length, and for this reason each of the active rolls 2 is generally placed either on two rollers 21, called intermediate rolls, which rest on three rows of back-up rolls, or directly on two rows of back-up rolls 22, all combinations being possible according to the specifications of the machine. Special stops are provided at the ends of the rolls to enable them to operate satisfactorily. The assembly consisting of an active roll and its carrying rollers is termed "levelling unit".
The levelling unit or units may be followed by devices which are intended to correct a transverse crossbow of the strip or a longitudinal camber; the active rolls of these devices are called: anti-crossbow roll 15 and anti-camber roll 16. These two latter functions are sometimes combined.
Deflector rollers 18, whose number depends on the arrangement of the machine, may be located at different places in order to cause the strip 3 to follow a particular path when passing through the machine, or to perform other functions.
The tangential speed of the rollers 17 of the exit block 12 is greater than that of the rollers 13 of the entrance block 11, so as to subject the strip to an adjustable elongation. Moreover, the positions of the levelling rolls 2, anti-crossbow rolls 15 and anti-camber rolls 16 are adjustable in height relative to the middle plane of advance of the strip, which thus follows a zig-zag path, which may be adjusted. The strip which is subjected to elongation and traction as it passes over each small-diameter active roll, and the adjustment in height of the active rolls, makes it possible to modify the distortion stress applied to the strip. The latter is thus distorted in the plastic range over each active roll, with a wrap angle, by undergoing an elongation, which confers on it the shape of a developable surface, each of the active rolls 14, 15 and 16 performing its own role.
In such installations, all the longitudinal fibers are brought to the same length in the plastic range and, for current materials, all defects in surface evenness, such as "long edges", "long centers", "pockets", etc., may be corrected.
In some cases, in order to improve the effect of correcting the surface evenness, it has already been proposed to provide the ends of certain rolls over which the strip advances with narrowed sections whose diameter progressively decreases relative to that of the central section of the roll so that the path traveled is longer in the central area than at the edges.
By thus causing paths to be traveled which have different lengths in the central area and at the edges of the strip which is subjected simultaneously to distortion in the elastoplastic range, elongations over the central area of the strip are increased during the levelling operation in a manner such that, after leaving the leveller, the fibers all have substantially the same length.
The narrowed sections may be provided at both ends of one and the same roll whose central section, forming the working bed and normally having a constant diameter, covers the central area of the strip over a constant width, the areas of the two edges passing over the two narrowed ends.
The result of this is that a roll of a specific profile may be suitable only for levelling strips whose width varies only to a small degree. Each roll is therefore given a width range and, when a strip of greater or smaller width than the set limits is to be processed, the roll must be replaced with another roll whose narrowed sections are separated from one another by the desired distance.
It is thus necessary to have different levelling rolls with central roll sections of different lengths that each corresponding to a width range, which rolls are exchanged according to requirements. This has drawbacks due, in particular, to the time required for the exchange.
In order to remedy this drawback, the use of rolls provided with a narrowed section at only one end has already been proposed, two successive rolls being equipped with narrowed sections located at opposite ends in a manner such that one edge of the strip passes over the narrowed section of one of the rolls, the other edge passing over the narrowed section of the other roll. By displacing the two rolls parallel to their axis and in opposite directions, on either side of the median longitudinal plane of the strip, it is possible to adjust the distance between the two narrowed sections according to the width of the strip.
However, such an arrangement, which is attractive in principle, creates problems in practice. In fact, even if such an arrangement enables the positioning of the narrowed sections to be adjusted according to the width of the strip, it is necessary to retain the possibility of easily replacing the working rolls, for example in the event of wear. Thus, intervention on the machine leads to a stoppage or, in any case, a curbing of production, and it must therefore be possible to perform this as rapidly as possible given the productivity of current installations which permit strip advance speeds of the order of 800 m/min to be attained.
Moreover, it is necessary to retain the possibility of adjusting the relative heights of the working rolls.
For this reason, it is generally preferred to separate the functions and to provide narrowed ends not on the active levelling rolls but on other rolls, for example the deflecting rollers which are changed less often and whose axial positions are more easily adjusted.
However, passage over the narrowed sections is more effective when the latter are provided on small diameter rolls such as the active roll, over which the strip is subjected to distortion in the plastic range.
However, difficulties in production and installation are then encountered, the more so since there is generally a restricted space available and since it is often necessary to improve existing installations.